1. Field of the Invention
The present invention relates to an automatic transmission for a vehicle and, more specifically, to an automatic transmission having a gear train comprising three single-pinion planetary gear sets.
2. Description of the Prior Art
As is generally known, the planetary gear set comprises three components, namely, a sun gear, a ring gear, and one or more pinion gears meshing with the sun gear and the ring gear. One of the three components is an input member, one of the rest of the components is an output member and the other component is a control member. The control member is fixed to rotate the output member at a rotational speed higher than that of the input member, to rotate the output member in a direction reverse to the direction of rotation of the input member at a rotational speed lower than that of the input member or to rotate the output member in the same direction as the input member at a rotational speed lower than that of the input member. Accordingly, the gear train for an automatic transmission, in general, comprises a plurality of planetary gear sets in combination. The speed change ratio of an automatic transmission is dependent on the configuration of the automatic transmission, namely, the manner of combination of a plurality of planetary gear sets, the gear ratio of each of the planetary gear sets (the ratio between the sun gear and the ring gear in the number of teeth) and the type of the planetary gear sets, namely, a single-pinion type or a double-pinion type. However, all the mechanically possible configurations are not necessarily practicable; the configuration of a practicable automatic transmission is dependent on various practical conditions including the possibility of mounting the automatic transmission on a vehicle, the possibility of manufacture, speed change characteristics and required power transmission performance. That is, almost unlimited option in determining the configuration of the gear train of an automatic transmission makes very difficult the creation of a gear train meeting requisite conditions of an automatic transmission for a vehicle.
Under such circumstances, various automatic transmissions have been proposed. Automatic transmissions each having three planetary gear sets have been disclosed in, for example, Japanese Patent Provisional Publication Nos. 60-88252, 51-48062 and 54132058.
An automatic transmission disclosed in Japanese Patent Provisional Publication No. 60-88252 is of five-forward-speed and one-reverse-speed having a gear train consisting of three single-pinion planetary gear sets. However, when the respective gear ratios of the planetary gear sets of this automatic transmission (the ratio between the number of teeth of the sun gear and that of the ring gear) are determined so that a sequence of the speed change ratios of the automatic transmission approximates a geometric progression to avoid sharp change of the driving force at the moment of gear change operation, the gear ratio of one of the planetary gear sets must be very large, which causes undesirable increase in the size of the planetary gear set. On the contrary, when the respective gear ratios of the planetary gear sets are determined so that the respective sizes of the planetary gear sets are not excessively large, it is impossible to set a sequence of speed change ratios substantially in a geometric progression and the power performance of the vehicle is unsatisfactory.
An automatic transmission disclosed in Japanese Patent Provisional Publication No. 51-48062 is of five-forward-speed and two-reverse-speed employing a combination of two single-pinion planetary gear sets and a double-pinion planetary gear set. This automatic transmission, however, requires the operation of four engaging means, namely, two clutch means and two brake means in changing gear from the first gear to the second gear, and from the second gear to the third gear; consequently, the automatic transmission increases shocks attributable to the gear change operation or requires unavoidably a complicated gear change control operation.
An automatic transmission disclosed in Japanese Patent Provisional Publication No. 54-132058 is of five-forward-speed and one-reverse-speed employing a combination of a Ravignawx planetary gear set and a single-pinion planetary gear set or a combination of two single-pinion planetary gear sets and a double-pinion planetary gear set. However, the sun gears of this automatic transmission are subjected to high torques when the first gear, the second gear or the reverse gear is selected, which is disadvantageous in respect of durability and strength of the automatic transmission. Furthermore, power circulation occurs in the automatic transmission when the third gear is selected adversely affecting the strength and durability of the automatic transmission. When one of the three brake means of the automatic transmission is omitted to simplify the construction, torque load on the clutch means increases to deteriorate the durability of the clutch means or the clutch means must be replaced with another clutch means having a greater torque transmission capacity. Still further, the automatic transmission needs a double-pinion planetary gear set.
U.S. Pat. No. 4,233,861 discloses an automatic transmission having not less than five forward speeds and one reverse speed. The gear train of this automatic transmission comprises a Ravignawx planetary gear set having first and second pinion gears, and a single-pinion planetary gear set. The first pinion gear of the Ravignawx planetary gear set meshes with a first sun gear and a ring gear, and the second pinion gear meshes with the first pinion gear and a second sun gear having a diameter smaller than that of the first sun gear. The first and second pinion gears are supported on a planetary carrier. The ring gear of the Ravignawx planetary gear set is interlocked with the ring gear of the single-pinion planetary gear set; the first sun gear is connected through a first clutch to an input shaft; the carrier of the Ravignawx planetary gear set is connected through a second clutch and an overdriving clutch to the sun gear of the single-pinion planetary gear set; the second sun gear is connected through a third clutch to the input shaft; the carrier of the Ravignawx planetary gear set is connected through a fourth clutch to another input shaft. The automatic transmission is provided with a first brake for holding the first sun gear, a second brake for holding the sun gear of the single-pinion planetary gear set, and a third brake for holding the carrier of the Ravignawx planetary gear set.
This automatic transmission disclosed in U.S. Pat. No. 4,233,861 has five forward speeds and one reverse speed. However, this automatic transmission requires a complicated gear change control in changing the gear from the third gear to the fourth gear without shocks, because the first clutch and the second brake must be disengaged and the second and fourth clutches must be engaged in changing the gear from the third gear to the fourth gear, and such gear changing operation is liable to cause shocks. Furthermore, the rotation of the second sun gear at a comparatively high rotating speed when the fifth gear, i.e., a gear for over-drive, or the reverse gear is selected may possibly increase the temperature of the engine oil or affects adversely to the durability of the automatic transmission.
The automatic transmission having three planetary gear sets may be relieved to some extent from these technical disadvantages by providing clutch means between some rotary components of the planetary gear sets to connect or disconnect the rotary components according to the operating mode of the automatic transmission.
U.S. Pat. No. 4,660,439 discloses an automatic transmission incorporating such an improvement. The gear train of this automatic transmission comprises three single-pinion planetary gear sets, namely, front, central and rear planetary gear sets, and is provided with a clutch for selectively connecting the sun gear of the front planetary gear set and the sun gear of the central planetary gear set, a clutch for selectively connecting the sun gear of the front planetary gear set and the carrier of the central planetary gear set, a clutch for selectively connecting the carrier of the central planetary gear set and the sun gear of the rear planetary gear set or a clutch for selectively connecting the carrier of the front planetary gear set and the sun gear of the central planetary gear set. Such a clutch increases the number of possible gears and avoids the rotation of some rotary component at an excessively high rotating speed. However, at least three clutches or brakes must be operated in changing the gear from the second gear to the third gear, or from the third gear to the fourth gear, which is liable to cause shocks and requires a complicated gear change control operation.